High-pressure Ca4Al6O13:: An example of a calcium aluminate with three different types of coordination polyhedra for aluminum

The crystal structure of tetracalcium trialuminate (Ca4Al6O13), synthesized at 1250 degrees C and 2.5 GPa, has been determined from single-crystal X-ray data by direct methods [space group Pcan, Z = 8, a = 5.3002(2) Angstrom, b = 17.7610(5), c = 21.0887(9) Angstrom] and refined to R1 = 6.42%. The unit cell parameters of Ca4Al6O13 exhibit a relationship to those of perovskite: a approximate to root 2 a(Pv), b approximate to 5 a(Pv), and c approximate to 4 root 2 a(Pv). The diffraction data showed the typical features of a pseudotranslational symmetry: all reflections (hkl) with l equal 4n (n is an integer) had significantly higher intensity than the reflections with l not equal 4n. Furthermore, diffuse streaks parallel to b* were observed. The new compound exhibits Al3+ in three different kinds of coordination polyhedra: octahedra, tetrahedra, and trigonal bipyramids. One of the two main building units is slightly corrugated sheets of perovskite-type corner sharing AlO6 octahedra perpendicular to [010]. The octahedral sheets are connected by layers containing tetrahedral zweier single chains. Within these layers the tetrahedral chains are linked by two different kinds of rods containing distorted trigonal bipyramids sharing common corners and edges, respectively. The tetrahedral chains and the bipyramidal rods are parallel to [100]. Charge compensation is achieved by the Ca ions, which are coordinated by 9 or 10 oxygen cations.